This invention relates to fuel elements for nuclear reactors, and more particularly it is concerned with a novel method of production of a composite billet for a fuel cladding tube of a composite cladding type comprising a zirconium alloy casing and a metallic liner, formed of zirconium, which is provided to the inner surface of the zirconium alloy cladding.
A nuclear fuel element for a power reactor now widely used includes a nuclear fuel material sealed in a tubular casing or cladding formed of noncorrosive, nonreactive and good thermal conductive material. A plurality of such fuel elements are assembled in predetermined spacing in lattice form to provide a fuel assembly. A suitable number of fuel assemblies are assembled into a conglomerate of a nuclear fission chain reaction type or a reactor core capable of carrying out a self-maintaining nuclear fission. The reactor core is placed in a reactor vessel through which a coolant flows.
The cladding serves a plurality of purposes. One of the principal objects is to prevent the occurrence of the chemical reaction between the nuclear fuel and coolant or between the nuclear fuel and moderator. The second object is to prevent radioactive nuclear fission products, which are partly in a gaseous state, from leaking from the fuel and from entering the coolant or moderator. The cladding material generally used nowadays is stainless steel or zirconium alloy.
A number of problems are encountered when a certain type of metal or alloy is used as a cladding material for producing nuclear fuel elements for use in a reactor core, because the cladding material shows a mechanical or chemical reaction under specific conditions. Zirconium and its alloys are excellent nuclear fuel cladding materials when acting under stationary state conditions. This is because of the facts that zirconium and its alloys have a small neutron-absorption cross section, and that they are, at a temperature of below about 400.degree. C., of high strength, ductile, very stable and nonreactive in the presence of pure water or steam generally used as coolant and moderator for a nuclear reactor.
On the other hand, the fuel elements behave in such a manner that, because of mutual action between the nuclear fuel, the cladding and fission products produced as a result of nuclear fission reaction, the cladding becomes brittle with the result that there is caused the fear of the occurrence of cracks. It has been ascertained that this undesirable behavior is promoted by mechanical stresses produced locally in the cladding due to the difference in the coefficient of thermal expansion between the fuel and the cladding. During operation of a nuclear reactor, fission products are released from the nuclear fuel as a result of fission reaction and remains on the surface of the cladding. In the presence of specific nuclear fission products such as iodine and cadmium etc., stress corrosion cracking occurs due to the action of local stresses and strain.
As a means for obviating these disadvantages, proposals have been made to provide a metallic barrier between the fuel and cladding, as disclosed in U.S. Ser. No. 838,161 dated Sept. 30, 1977 which has issued as U.S. Pat. No. 4,200,492 and U.S. Ser. No. 522,856 dated Nov. 11, 1974, for example. In these proposals made in the past, a proposal to use a composite type cladding including a sheet of zirconium of suitable purity attached in metal-to-metal bonding to the inner surface of a zirconium alloy casing as a metallic liner is expected to achieve the best result. In such proposal, the zirconium liner has a thickness which is about 5-30% of that of the cladding. As compared with zirconium alloys, zirconium is better able to remain in a soft state during irradiation by neutrons, so that it reduces local stresses in the nuclear fuel element and protects the cladding from stress corrosion cracking. An additional feature of zirconium is that it does not involve the problems of neutron capture penalty, heat transfer penalty and noncompatibility of materials. Besides zirconium, nickel, iron and copper may also be used to provide a metallic barrier.
A composite type cladding of the aforesaid construction has generally been fabricated as described hereinbelow. As shown in a chart in FIG. 1, a zirconium ingot for forming a metallic barrier and a zirconium alloy ingot for forming a cladding are prepared by the melting of briquet, and a hollow zirconium billet is inserted in a hollow zirconium alloy billet to provide a unitary composite billet, which is then extruded at an elevated temperature in the range between about 500.degree. and 750.degree. C. by a usual hot extrusion method to produce an extruded composite tube. The extruded composite tube is subjected to usual tube contraction by use of cold rolling process. Thus a composite tube having a cladding of desired dimensions is obtained.
In the cladding tube of a composite type, it is essential that the zirconium barrier have the desired dimensions and that a satisfactorily metallurgical bonding be formed between the zirconium liner and the cladding material over the entire area. To this end, it is necessary that the hollow zirconium billet (inner member) is integrated metallurgically to the hollow zirconium alloy billet (outer member) even in the state of a billet (that is, in the stage before hot extrusion), and that such metallurgical integration formed in the composite billet be maintained during the subsequent extrusion and tube contraction operations in which the billet undergoes deformation. The following methods have generally been used for obtaining such composite billet in which the inner and outer members are metallurgically integrated.
(1) The inner member is inserted in the outer member, and they are bonded together by explosion bonding.
(2) Following insertion of the inner member in the outer member, the composite billet is heated to effect diffusion bonding to obtain an integrated structure.
Since a fuel cladding tube requires very high reliability in performance, some disadvantages are caused with respect to the above-described conventional methods. More specifically, the method of (1) can not be applied to the production of a composite tube of small inner diameter (such as below about 30 mm). Further, in the explosion bonding, bonding between the outer and inner members does not become uniform in the direction of the axis of the composite billet and there occur other problems such as a riffle-like bonding interfaces being caused. The conventional method (2) requires the heating of the members to be bonded at elevated temperature over a prolonged time (at 750.degree. C. for eight hours, for example). Moreover, the need to apply pressure during heating or to mechanically effect preliminary bonding so as to obtain uniform diffusion renders the process steps complex and makes it difficult to keep the operation condition at a suitable level. An additional disadvantage is that unless a vacuum or inert atmosphere is used, oxidation of the interface is unavoidable.
In order to reduce the dimensions of the composite billet produced by the aforesaid processes to obtain a fuel cladding tube of the usual dimensions, the composite billet must be worked in a high temperature atmosphere by hot working such as hot extrusion. Thus, there is caused the oxidation of the interface while the composite billet is being subjected to hot extrusion or other treatment, so that the formation of oxides at the interface reduces the bonding strength. Generally, the inner and outer members for forming a composite billet are machined before being assembled into a unitary structure, so that irregularities of several microns are originally present on their surfaces. Thus the problem of the bond strength being reduced due to formation of oxides at the interface has been unavoidable.